This invention relates generally to water chillers, and more particularly to a system providing precision-control of process water temperature over a broad range of loads.
Water is widely used as a coolant for equipment used in various processes. It is used in rubber and plastics processing, calendaring, coating, printing, chemical processing, laminating, and many other manufacturing processes. Injection molding machines and industrial laser machines are examples where intermittent cooling loads, and substantial variations in cooling loads, must be handled. In addition, manufacturers and users of such equipment find that better performance and process quality can be achieved if coolant temperatures are stable.
Typical chillers of thirty tons and less are designed to have the refrigeration capacity sufficient to handle the largest cooling loads that will be imposed on them. Refrigerating systems having reciprocating compressors are typically provided with condenser by-pass paths or compressor unloading systems to avoid excessive cooling during light load conditions. These are typically designed to reduce the system cooling performance approximately 50 percent. Further reductions in cooling performance below 50%, particularly in systems under twenty tons, usually are not done by compressor unloading or hot gas by-pass techniques, and may ultimately require shutting down the compressor.
U.S. Pat. No. 3,859,812 to Pavlak discloses the use of cylinder unloading and hot gas by-pass to reduce refrigeration performance in a cooling system for machine tool coolant. U.S. Pat. No. 4,546,618 to Kountz et al. discloses a complex capacity control system for refrigeration in a water chiller, using compressor speed and vane control. The Kayma U.S. Pat. No. 4,502,289 discloses cold water supply systems with supply and return tanks and cold water temperature and level sensing and a computer 80 to control pumps, valves, and refrigerators for water temperature control. It refers to prior art FIGS. 1 and 2 disclosing return cold water temperature measured by sensor 22 in return water tank 24, and computer 14 responding to control capacity of turbo refrigerators 20 by using their automatic vane control feature. The asserted improvement involves mixing suitable amounts of water directly from the return tank and from the refrigerators, in the supply tank.
Cycling of compressors is detrimental to the compressors and associated equipment. It may also have a negative impact on the electrical power factor of the manufacturing plant. Large temperature swings of the chilled water usually result when temperature control is attempted by the starting and stopping of the compressor. Such swings, of 5.degree. F. or more, can be intolerable in processing equipment. The present invention is the result of efforts to provide a stable cooling fluid temperature at light loads as well as heavy loads and at various loads between light and heavy, and without detriment to the chiller system.